1. Field of the Invention
The present invention is directed to a measuring instrument for intracardial acquisition of the blood oxygen saturation of a patient, and in particular to such a measuring instrument for use in controlling the pacing rate of a heart pacemaker implanted in the patient.
2. Related Application
The subject matter of the present application is related to the subject matter of copending application Ser. No. 051,857 (Roland Heinze and Hakan Elmqvist) filed May 20, 1987.
3. Description of the Prior Art
German OS No. 31 52 963 discloses a measuring probe for generating a signal corresponding to the blood oxygen saturation of a patient including a measuring current path with a light transmitter and a light receiver arranged such that the light receiver receives light emitted by the light transmitter and reflected by the blood. This known device undertakes a useful signal measurement and a reference measurement independent of the blood reflection through the measuring probe. The measuring probe is connected to an evaluation circuit through two lines, the evaluation circuit charging the measuring probe with a current or with a voltage, and thereby permitting separate evaluation of the signals arising during useful signal measurement and reference signal measurement. The light emitter in this known device is a light emitting diode, and the light receiver is a phototransistor. The light emitting diode and the phototransistor are connected in parallel such that the conducting state current through the light emitting diode is superimposed with the current through the phototransistor caused by the incident light. If the measuring probe is driven with a constant current or with a constant voltage, the light reflected by the blood, dependent on the blood oxygen saturation thereof, triggers a current flow in the phototransistor which effects a current (or voltage) modification at the measuring probe. The voltage or current modification generated by the light reflection is identified in an evaluation circuit by comparing the measured signal, which is obtained when the light emitting diode and the phototransistor are driven, with a reference signal. The reference signal is formed by a pulse of the same operating voltage, but having an inverted operational sign in comparison to the voltage used for the useful signal measurement. This pulse is supplied through a diode connected with opposite plurality to the pularity of the light emitting diode. The operating characteristics of the diode in the reference circuit and the characteristics of the light emitting diode are preferably identical.
In this known measuring instrument, therefore, only two electrical leads are necessary for obtaining the useful signal measurement and the reference measurement. This is an advantage because such leads must be accomodated in a catheters having the smallest possible diameter and great flexibility, both of which are decreased by the presence of more electrical leads. Moreover, every additional electrical lead increases the probability of a failure.
A disadvantage of this known device, however, is that the voltage used for obtaining the measured signal must be reversed in polarity in order to make the reference measurement. Given the standard format for the voltage supply of heart pacemakers, wherein one pole of the supply voltage is rigidly connected to the housing, a substantial circuit outlay is required in order to make this polarity reversal. Additionally, the same current is used for the reference measurement as for the useful signal measurement.
Other commercially available devices are known wherein an infrared emitting diode is connected for making the reference measurement, with the receiver remaining in operation during the reference measurement as well. The wavelength of the infrared emitting diode is selected such that the reflection of the blood is independent of its oxygen saturation. A reference measurement is thereby obtained which permits deposits on the measuring probe to be taken into account.